As a method for performing communication between devices, serial communication and parallel communication are known. In the parallel communication, data of a plurality of bits are transmitted/received simultaneously using a plurality of signal lines, while in the serial communication, data is transmitted/received bit by bit using a single signal line. As is clear from the difference between two communication methods, the serial communication has an advantages that its simpler structure allows longer distance communication than the parallel communication.
As a conventional technique relating to the present invention, there is known a slave stations simultaneous start-up method that outputs, to a plurality of slave stations that control their corresponding devices, control data and a start-up signal for use in the slave stations to thereby start up the slave stations simultaneously (refer to, e.g., Patent Document 1).    [Patent Document 1] Japanese Laid-open Patent Publication No. 07-75357
However, in order to control a plurality of devices in 1:n serial communication, it is necessary to issue a control instruction for each transmission destination in series. That is, it is necessary to communicate with one device after waiting for a time required for communication with a previous device. Hereinafter, with reference to FIGS. 20 and 21, problems in the serial communication will be described.
FIG. 20 is a view illustrating a control system using conventional serial communication, and FIG. 21 is a flowchart illustrating operation of a master device in a communication system using conventional serial communication.
As illustrated in FIG. 20, a control system using conventional serial communication includes a working device 1, a master device 2 that instructs the working device 1 to perform work, and a serial cable 3 that connects the working device 1 and master device 2. The working device 1 includes a leading end portion 1a having a predetermined function and motors 1x to 1z for moving the leading end portion 1a in three axial directions denoted by X, Y, and Z arrows. Not illustrated control devices are connected to the motors 1x to 1z, and the master device 2 transmits an operation command to the control devices serving as slave devices. When moving the leading end portion in the three axial directions, the master device 2 that transmits an operation command to the not illustrated control devices controlling the motors 1x to 1z performs processing as illustrated in FIG. 21.
The master device 2 transmits a command to the control device corresponding to the motor 1x (S801) and then receives a reply to the command from the control device of the motor 1x (S802). Then, the master device 2 transmits a command to the control device corresponding to the motor 1y (S803) and then receives a reply to the command from the control device of the motor 1y (S804). Finally, the master device 2 transmits a command to the control device corresponding to the motor 1z (S805) and then receives a reply to the command from the control device of the motor 1z (S806).
As described above, the axes of the working device 1 do not move simultaneously. That is, the working device 1 operates not simultaneously but sequentially with a time interval required for communication every time the axis to be moved is switched. Each of the control devices of the motors 1x to 1z has a previously assigned selection address which is a unique address and, for execution of the received command, determines whether the selection address added to the command coincides with the selection address assigned to the control device itself.
The time difference caused among the motors 1x to 1y may pose a problem that the leading end portion 1a cannot move on a trajectory of an intended curve or straight line. Further, when a device to be controlled, not only the motor, needs to be brought into an emergency stop, it is necessary to repeatedly and individually transmit the same command to control devices corresponding to the devices to be controlled. Further, a time period required for the last device to be started-up is increased in proportion to the number of devices to be controlled.
The technique disclosed in Patent Document 1 is a technique in which a main control device simultaneously starts up devices to be controlled by transmitting a control command for making the devices to be controlled work and a simultaneous start-up request to a plurality of slave stations (control devices) by multicasting. Thus, in the case of an inexpensive low speed communication system, the multicasting of the control command and simultaneous start-up request to a plurality of slave stations makes a time period from when the main control device issues the start-up request to when it actually makes the devices to be controlled work longer. Further, the multicasting of the start-up signal is necessary in the case where only one slave station is started up, which is disadvantageous in an environment where control is often made for a single slave station.